Acid cleaning operations are commonly employed to remove adhering substances, such as mill scale and fly ash, from the interior surface of vessels, tubes and related industrial processing equipment, particularly where such equipment is fabricated from ferrous metals. Aqueous processing equipment, especially that operating at elevated temperatures, tends to acquire a build-up of insoluble deposits. Typically, acid cleaning is used to remove these deposits. For example, acid cleaning is employed to remove lime deposits or water scale from power plant boilers and piping systems and evaporating equipment. Likewise, acid cleaning is employed to remove scale and deposits from processing equipment in such plants as refineries, utility companies, paper mills, chemical plants, and the like. Since cleaning acid tends to remove a portion of the basis metal of the industrial equipment with each cleaning, the use of inhibitors to minimize basis metal loss in acid cleaning can substantially extend the life of the equipment.
Similar acid cleaning solutions are also used in acid pickling for the removal of undesirable oxide coatings from metals (usually ferrous metals) before subjecting them to further treatment such as phosphate coating, enameling, electroplating and the like.
The art recognizes a variety of acids for chemical cleaning such as, for example, inorganic acids such as hydrochloric, sulfuric, nitric and phosphoric acids, and organic acids such as formic, citric, and alkylene polyamine carboxylic acids.
Although the complexity of the inhibition phenomena is such that there are no particular criteria by which one can predict the inhibiting power of degree of inhibition that can be achieved with any particular inhibitor, a variety of inhibitors for acid cleaning systems are well known in the art. Despite the fact that there does not appear to be any recognized basis for correlating chemical structure to inhibiting strength, a number of suitable inhibitors have been found among the organic amines and the organic sulfur-containing compounds. Sulfur-free inhibitors are increasingly preferred since sulfur-containing compounds may leave a residue of polythionic acid on the basis metal of the equipment being cleaned. Polythionic acid is known to produce metal pitting.
Several aromatic amines are known to be useful as inhibitors. By way of illustration, U.S. Pat. No. 2,692,191 discloses alkyl-substituted mercaptobenzimidazoles as inhibitors for unsaturated mineral oil, such as cracked gasoline, against corrosion of metals. U.S. Pat. No. 3,091,591 discloses cyclic saturated amines for non-oxidizing alkaline and acidic corrodant systems to inhibit the corrosion of ferrous metals. U.S. Pat. No. 2,671,807 discloses a process for producing N-dialkyl-substituted aromatic amines which are mentioned as being useful as corrosion inhibitors for metal surfaces. U.S. Pat. No. 2,724,695 discloses compounds as metal inhibitors in natural oil-brine mixtures which contain both an imidazoline ring and a imidazolidine ring. U.S. Pat. No. 2,727,003 discloses imidazoline ring containing compounds as inhibitors for natural petroleum-oil brine mixtures to reduce their corrosive action upon oil field equipment.
Though it is common practice to blend two, three or more known inhibitors in an effort to provide acid inhibition suitable to specific conditions, the instances where such blends produce an inhibitor combination of unexpectedly enhanced inhibition strength are rare. Those few synergistic combinations which are known generally involve an amine as one of the essential components. One of the earliest known examples of synergism in acid inhibitors, is the amine/thiourea combination. When used in sulfuric acid as a pickling inhibitor, amines are not effective and thioureas have only moderate value. However, the combination of amines and thioureas produces an effective inhibitor combination for sulfuric acid pickling. Another instance in which an amine is essential is the combination of acetylenic alcohols and nitrogen compounds in hydrochloric acid. Both inhibitors are only reasonably effective when used alone, but together they provide an excellent inhibitor. Still another example of a synergistic combination also involving an amine can be found in U.S. Pat. No. 3,382,179 wherein an acetylenic compound, an amine and naphthenic acid are combined to provide a synergistic combination.
It has now been found that gelatin together with a compound selected from a specific class of aromatic amines provides particularly effective inhibition for aqueous carboxylic acid solutions in the cleaning of metal industrial equipment.